Silicon is a material which is used more and more often in the manufacture of mechanical parts and in particular of micro-mechanical parts, both “captive” parts i.e. parts which stay connected to a substrate on which they have been etched, or “free” parts such as parts belonging to the kinematic chain of a horological movement.
Compared to metals or metal alloys conventionally used for manufacturing micro-mechanical parts, such as toothed wheels, articulated parts or springs, silicon has the advantage of having a density that is 3 to 4 times lower and therefore of having a very reduced inertia and of being insensitive to magnetic fields. These advantages are particularly interesting in the horological field both for isochronism and the operating duration of the timepiece when the energy source is formed of a spring.
Silicon is however known to be sensitive to shocks, which may be necessary during assembly, inevitable in operation or accidental when for example the user knocks his wristwatch against something or drops it.
EP patent No 1 422 436 discloses a silicon hairspring formed of a spiral shaped bar coated over its entire surface with a layer of amorphous silicon oxide. According to this document, the first thermal coefficient of Young's modulus for silicon oxide is opposite to that of silicon. Thus, the combination of a core made of silicon with an external coating of oxide is said to allow a reduction in said first thermal coefficient.
This prior art document does not mention the problem of shock sensitivity of parts made of silicon.